Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfer and fixing. Ink jet printing mechanisms can be categorized by technology as either drop on demand ink jet (DOD) or continuous ink jet (CU).
The first technology, “drop-on-demand” ink jet printing, provides ink drops that impact upon a recording surface by using a pressurization actuator (thermal, piezoelectric, etc.). One commonly practiced drop-on-demand technology uses thermal actuation to eject ink drops from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently to boil, forming a vapor bubble that creates enough internal pressure to eject an ink drop. This form of inkjet is commonly termed “thermal ink jet (TIJ).”
The second technology commonly referred to as “continuous” ink jet (CIJ) printing, uses a pressurized ink source to produce a continuous liquid jet stream of ink by forcing ink, under pressure, through a nozzle. The stream of ink may be perturbed in a manner such that the liquid jet breaks up into drops of ink in a predictable manner. Printing occurs through the selective deflecting and catching of undesired ink drops. Various approaches for selectively deflecting drops have been developed including the use of electrostatic deflection, air deflection and thermal deflection mechanisms.
In a first electrostatic deflection based CIJ approach, the liquid jet stream is perturbed in some fashion causing it to break up into uniformly sized drops at a nominally constant distance, the break-off length, from the nozzle. A charging electrode structure is positioned at the nominally constant break-off point so as to induce a data-dependent amount of electrical charge on the drop at the moment of break-off. The charged drops are then directed through a fixed electrostatic field region causing each droplet to deflect proportionately to its charge. The charge levels established at the break-off point thereby cause drops to travel to a specific location on a recording medium or to a gutter, commonly called a catcher, for collection and recirculation. This approach is disclosed by R. Sweet in U.S. Pat. No. 3,596,275 issued Jul. 27, 1971, Sweet '275 hereinafter. The CIJ apparatus disclosed by Sweet '275 consisted of a single jet, i.e. a single drop generation liquid chamber and a single nozzle structure. A disclosure of a multi-jet CIJ printhead version utilizing this approach has also been made by Sweet et al. in U.S. Pat. No. 3,373,437 issued Mar. 12, 1968, Sweet '437 hereinafter. Sweet '437 discloses a CIJ printhead having a common drop generator chamber that communicates with a row (an array) of drop emitting nozzles each with its own charging electrode. This approach requires that each nozzle have its own charging electrode, with each of the individual electrodes being supplied with an electric waveform that depends on the image data to be printed. This requirement for individually addressable charge electrodes places limits on the fundamental nozzle spacing and therefore on the resolution of the printing system.
A second electrostatic deflection based CIJ approach is disclosed by Vago et al. in U.S. Pat. No. 6,273,559 issued Aug. 14, 2001, Vago '559 hereinafter. Vago '559 discloses a binary CIJ technique in which electrically conducting ink is pressurized and discharged through a calibrated nozzle and the liquid ink jets formed are broken off at two different time intervals. Drops to be printed or not printed are created with periodic stimulation pulses at a nozzle. The drops to be printed are each created with a periodic stimulation pulse that is relatively strong and causes the ink jet stream forming the drops to be printed to separate at a relatively short break off length. The drops that are not to be printed are each created with a periodic stimulation pulse that is relatively weak and causes the drop to separate at a relatively long break off length. Two sets of closely spaced electrodes with different applied DC electric potentials are positioned just downstream of the nozzle adjacent to the two break off locations and provide distinct charge levels to the relatively short break off length drops and the relatively long break off length drops as they are formed. The longer break off length drops are selectively deviated from their path by a deflection device because of their charge and are deflected by the deflection device towards a catcher surface where they are collected in a gutter and returned to a reservoir for reuse. Vago '559 also requires that the difference in break off lengths between the relatively short break off and the relatively long break off length be less than a wavelength (λ) that is the distance between successive ink drops or ink nodes in the liquid jet. This requires two stimulation amplitudes (print and non-print stimulation amplitudes) to be employed. Limiting the break off length locations difference to less than λ restricts the stimulation amplitudes difference that must be used to a small amount. For a printhead that has only a single jet, it is quite easy to adjust the position of the electrodes, the voltages on the charging electrodes, and print and non-print stimulation amplitudes to produce the desired separation of print and non-print droplets. However, in a printhead having an array of nozzles parts tolerances can make this quite difficult. The need to have a high electric field gradient in the droplet break off region makes the drop selection system sensitive to slight variations in charging electrode flatness, electrode thicknesses, and electrode to jet distances that can all produce variations in the electric field strength and the electric field gradient at the droplet break off region for the different liquid jets in the array. In addition, the droplet generator and the associated stimulation devices may not be perfectly uniform down the nozzle array, and may require different stimulation amplitudes from nozzle to nozzle to produce particular break off lengths. These problems are compounded by ink properties that drift over time, and thermal expansion that can cause the charging electrodes to shift and warp with temperature. In such systems, extra control complexity is required to adjust the print and non-print stimulation amplitudes from nozzle to nozzle to ensure the desired separation of print and non-print droplets. B. Barbet and P. Henon also disclose utilizing break off length variation to control printing in U.S. Pat. No. 7,192,121 issued Mar. 20, 2007.
B. Barbet in U.S. Pat. No. 7,712,879 issued May 11, 2010 discloses an electrostatic charging and deflection mechanism based on break off length and drop size. A split common charging electrode with a DC low voltage on the top section and a DC high voltage on the lower segment is utilized to differentially charge small drops and large drops according to their diameter.
T. Yamada in U.S. Pat. No. 4,068,241 issued Jan. 10, 1978, Yamada '241 hereinafter, discloses an inkjet recording device which alternately produces large drops and small drops. All drops are charged with a DC electrostatic field in the break off region of the liquid jet. Yamada '241 also changes the excitation drop magnitude of small drops not necessary for recording so that they will collide and combine with the large drops. Large drops and large drops combined with small drops are guttered and not printed while deflected small drops are printed. One of the disadvantages of this approach is that deflected drops are printed which could result in drop placement errors. This approach is very sensitive to small changes in stimulation amplitude and to small changes in ink properties. Furthermore, as the smaller drop needs to be much smaller than the larger drop in order to be able create different charge states on each; higher nozzle diameter nozzles are required for producing the desired sizes of print drops. This limits the density of nozzle spacing that can be utilized in such an approach and severely limits the capability to print high resolution images.
As such, there is an ongoing need to provide a continuous printing system that electrostatically deflects selected drops, is tolerant of drop break off length, has a simplified design, and yields improved print quality.